Integrated semiconductor memories have a plurality of data lines through which data are read out from memory cells or are written to the memory cells. With the aid of the data lines it is therefore possible to communicate data, that is to say a plurality of digital bits, between the memory cells and external terminals of the semiconductor memory. On these data paths provision may furthermore be made for signal amplifiers, for example, sense amplifiers, by means of which the electrical potentials of two mutually complementary bit lines are spread, or downstream secondary signal amplifiers which, as output amplifiers, once again amplify the communicated signals before the latter reach the output terminals of the semiconductor memory. The data lines can be provided, in particular, within the path section between the sense amplifiers and the output amplifiers.
An integrated semiconductor memory can be, for example, a volatile semiconductor memory, for instance a DRAM (dynamic random access memory) or a nonvolatile semiconductor memory. The data are stored in memory cells connected to first and to second lines, which are usually referred to as word lines and bit lines. In the case of a DRAM, the memory cell can comprise, for example, a selection transistor and a storage capacitor, wherein the selection transistor may be formed as a MOSFET (metal oxide semiconductor field effect transistor), the gate electrode of which is part of a word line and the first source/drain region of which is connected to a bit line.
The functioning of a semiconductor memory presupposes a proper electrical contact between the relevant conductive structures. Due to the dictates of production, however, deviations always occur with regard to the relative position of the conductive and also insulating structures with respect to one another, the deviations having disadvantageous effects on the electrical properties, to be precise also on the switching properties within the semiconductor memory.
Thus, between structures to be deposited one on top of another, in particular lateral offsets in the lateral direction, i.e., parallel to the substrate surface, can lead to relatively high-impedance electrical connections or even connection interruptions between the structures. As a result of lithographic misalignments (overlay misalignment), in particular, a lateral offset often arises between structures deposited one on top of another. One example of such lateral misalignments is the contact hole fillings (vias) which are often required in the case of segmented word lines and that are intended to produce an electrical connection having the lowest possible impedance between the main word line and the respective word line segment, but themselves have only a small basic area. A slight lateral deviation of their position therefore leads rapidly to high-impedance or even interrupted and therefore unusable word lines. Comparable problems occur also in the case of bit lines, other lines, or in the case of other conductive structures.
Integrated semiconductor memories therefore have redundant memory areas, which can be activated as an alternative if, in the course of testing the semiconductor memory prior to its delivery, it is ascertained that individual memory areas which are automatically addressed during normal operation are defective. By way of example, individual word lines or bit lines may be prone to floating and therefore uncontrollable electrical potentials since their electrical connection is deficient or entirely interrupted. Problems can therefore occur when reading out from memory cells which are connected to the rest of the word lines or bit lines These problems are only avoidable if the corresponding word line or bit line is permanently deactivated and is replaced by a redundant word line or bit line. In this case, during later operation, the address of the respective word line or bit line will generally be unchanged, but it is ensured by permanent settings within the semiconductor memory, for example, with the aid of fuses or antifuses, that the signals intended for the associated address are rerouted to a memory area that is permanently activated as an alternative (for instance to a redundant word line or bit line).
Semiconductor memories therefore have memory areas which are formed as an alternative and are permanently activated instead of remaining as defective memory areas only after fixed programming. These redundant memory areas are usually integrated overall into the memory cell array. They require additional substrate area on the semiconductor substrate, but reduce the reject rate of the semiconductor chips during fabrication because the redundant memory areas can be used as an alternative in the event of defective memory areas having been identified.
A memory cell array is usually subdivided into a plurality of subunits. One possible subdivision mentioned here by way of example is the separation of a memory cell array or of a subsection thereof into a plurality of “memory segments”, wherein a memory segment can be understood to be, in particular, that memory area whose read-out data are all conducted to the same output amplifier. In the case of a DRAM, for example, the memory segment comprises those bit lines and those word line sections at which are arranged the memory cells whose data are communicated to, in each case the same output amplifier during read-out, via the sense amplifiers and data lines disposed downstream of the sense amplifiers. The word lines can also extend beyond a memory segment, such that only individual segments or partial areas of the word lines are assigned to the respective memory segment and thus to the respective output amplifier or secondary signal amplifier.
A memory cell array therefore contains memory segments defined by word line sections and bit lines (or if appropriate only by sections of bit lines). Such a memory segment is driven by a predetermined address range of memory addresses, for example, by a specific address space of the word line addresses and the bit line addresses. If a partial area of the respective memory segment is defective, it must be replaced by a redundant partial area.
Redundant memory areas are usually arranged between memory segments that are adjacent to one another. By way of example, it is possible to provide redundant memory cells which can be driven by redundant bit lines which can be read and/or written to by redundant sense amplifiers and redundant data lines disposed downstream of the sense amplifiers. The redundant memory cells can be connected to the same word lines as the rest of the memory cells of the respectively adjacent memory segments. Conversely, the memory cells can also be connected to redundant word lines but the same bit lines as adjacent memory segments, or be connected both to redundant word lines and to redundant bit lines.
Redundant data lines are used for reading out from redundant memory cells. In the case of a DRAM, the redundant data lines will be connected to the (redundant) sense amplifiers in the same way as the remaining, non-redundant data lines are connected to the regular, non-redundant sense amplifiers. In other types and designs of semiconductor memories, redundant memory areas including the redundant data lines are often arranged between the memory segments. The data contents of the memory segments are read out by means of regular, non-redundant data lines.
Additional substrate area is required for each redundant data line and the memory cells assigned thereto. Therefore, on the one hand endeavors are made to keep the number of redundant data lines and other redundant structures of the memory cell array as small as possible. On the other hand, the proportion of defective semiconductor chips which are still repairable by exchanging non-redundant structures for redundant structures is all the greater, the more redundant structures (for example, data lines) there are in the semiconductor memory.
One possible integration scheme would be to provide, between in each case two memory segments each containing a predetermined number of non-redundant data lines, in each case one or a plurality of redundant data lines which are assigned in each case to one of the adjacent memory segments. In this way, in the respectively adjacent memory segments, a data line could be replaced by the respective redundant data line. Thus, one or two redundant data lines running laterally alongside the regular data lines of the relevant memory segment can additionally be provided, for example, for each memory segment.
All semiconductor chips in which per memory segment one or two data lines (or a number of data lines corresponding to the number of redundant data lines per memory segment) are connected to defective memory areas would be repairable in this way. However, as soon as the number of non-redundant data lines which are assigned to defective or partly defective memory areas in at least one memory segment is greater than the number of redundant data lines per memory segment, such that a semiconductor chip can no longer be operated properly and has to be rejected.